In the agricultural market the aim of plant breeding is largely to increase the yield of the crops whereas in breeding of ornamental plants it is to produce new, different and attractive varieties. Whatever the final product, the goal of all breeders is to combine favourable characteristics of parent plants in the progeny.
Although Mendelian variation may result in the production of new/different characteristics and spontaneous mutations may result in the presence of sports, the most satisfactory method for the introduction of new characteristics is by carefully controlled breeding. With plants which are self-pollinating, this may take the form of pure-line selection, mass selection or hybridisation followed by handling of the segregating generations by a) pedigree method, b) bulk method or c) back cross method. These three methods are all based on the fact that selfing, or back-crossing to a homozygous parent leads to homozygosity, thus ensuring the presence of the traits in successive generations. With plants which are outbreeding self-pollination may occasionally be possible, but such usually results in a loss in vigour of the plants. Outbreeding plants may be bred using pure-line selection and hybridisation, with the later handling being the same as for the self-pollinated plants.
Back-crossing provides a precise way of improving varieties which are superior in a large number of characteristics and deficient in only a few. If such a method is employed in interspecific hybridisation, wherein the parent plants originate from different species, certain features of one species may be transferred to the other without impairing the taxonomic integrity. Thus one species may become enriched with characteristics originating from another.
There are often, however, problems associated with interspecific hybridisation. For example interspecific hybrids often suffer a loss of reproductive capacity with the result that F.sub.1 and later generations demonstrate a greater or lesser extent of sterility. Such may be due to genetic or cytoplasmic incompatibilities which are demonstrated either by a failure in fertilisation or death of a zygote before maturity.
With regard to ornamental plants, and in particular the members of the highly popular Pelargonium genus, during interspecific hybridisation between some members of the Ciconium section, such as P. acetosum, P. inquinans, and P. zonale, and the sole species of the Dribrachya section, P. peltatum, even though fertilisation may be accomplished, fully developed seeds have not resulted [Yu, Sun Nam (1985) Diss. Institut fur Landwirtschaftlichen und Gartnerischen Pflanzenbau, Weihenstephan]. Thus production of a fertile Pelargonium peltatum plant capable of being propagated by seed and illustrating a combination of the characteristics of the parent species, or the transfer of characteristics of one species to Pelargonium peltatum to produce such a fertile, seed propagatable plant, has thus far not been possible at the diploid level.
The genus Pelargonium belongs to the vegetable family Geraniaceae and is divided into 14 sections totalling approximately 280 species. Members of the Pelargonium genus are normally protandrous, i.e. the pollen develops before the stamen, and thus cross-pollination is the norm. An enormous variety of characteristics is encompassed by the members of the genus with plants ranging from trawling or hanging species to compact shrubs, from those with large and abundant flowers to others which produce few delicate flowers. The plants themselves are extremely popular with the public, being easy to care for and inevitably producing flowers of brilliant colour.
The present invention is largely concerned with the introduction of new reproducible traits into the species Pelargonium peltatum. The colours obtainable in the flowers of botanical species are predominantly shades of palish pink, lavender, mauve and sometimes whitish. Although scarlet, red, salmon and rose colours have been produced in some Pelargonium peltatum varieties, at both the diploid and tetraploid level, this plant material has a low fertility at the tetraploid level and is completely sterile at the diploid level. The presence of the genes resulting in these colours is probably the result of an interspecific hybridisation. Zehner et al (1981 Research Report Michigan State University) have shown that it is these colours which are most favoured by the public in pelargoniums and thus it would be economically advantageous to be able to produce such colouration in plants which are diploid, fertile and may be propagated by seed.
Due to the hermaphroditic nature of the botanical pelargonium plants, any plant may be selected as either the male or the female parent. In F.sub.1 hybrid seed production in a number of horticultural crops, pollination is normally accomplished by hand. However, in order to ensure that the proper cross is made between the desired male and female plants, it is often necessary to remove the male reproductive parts from the plant such that pollen from the female does not self-pollinate or pollinate a sibling female plant. Such physical removal of the anthers is understandably labour-intensive and costly. In Pelargoniums, genetic male sterility (which is caused by a defect in or absence of the required nuclear genes) in a female fertile plant is known in the species Pelargonium x hortorum of the section Ciconium. Such sterility is characterised by the absence of stamen--the male reproductive organs--but the presence of a complete female reproductive capacity. Cytoplasmic sterility, caused by the absence of both the fertility genes in the cytoplasm and the fertility restorer genes in the nucleus, is also known in some Pelargonium x hortorum varieties. Cytoplasmic male sterility is preferred by plant breeders, as its presence enables 100% of the motherline, compared with 50% of the genetic male sterile motherline, to be used for seed production. Further, with cytoplasmic male sterility it is possible to produce an F.sub.1 hybrid which is 100% male sterile. These male sterile plants are expected to produce flowers in more abundance than male fertile plants. Thus it would clearly be advantageous to plant breeders to have the feature of male sterility, and preferably cytoplasmic male sterility, in P. peltatum plants.
The diploid character (in P. peltatum 2N=18) is most favoured by plant breeders due to problems inherent in the propagation by seed of tetraploid material which is both genetically and cytologically more complex. The assurance of fertility in F.sub.1 and further generations produced from tetraploid parents can not be absolute with problems possibly being attributable to unequal segregation of chromosomes at meiosis, partial multivalent formation or lagging of chromosomes. Further, to reach a sufficient level of homozygosity in a tetraploid plant requires considerably more generations and it is thus preferable to use diploid material in such breeding programmes.